Turbine engines have at least one circular array of blades mounted around the circumference of a rotor disk. Each blade can be mounted by forming a mounting platform on the shank of the blade, in which the platform has a dovetail geometry that slides axially into a matching slot in the disk. U.S. Pat. No. 5,147,180 shows an example of a platform having an inverted “fir tree” geometry with multiple lateral teeth of descending width that is sometimes used.
The blade and platform may be cast integrally of an advanced single crystal superalloy such as CMSX-4 or PWA1484. However, casting the blade and platform in one piece has disadvantages. The size of the hole in the baffle through which the casting is withdrawn during the single crystal solidification process is dictated by the largest cross-sectional area of the part, which is usually the platform in the case of an integrally cast blade. The thermal gradient is not optimized when the baffle does not closely fit around the casting and can lead to the formation of casting defects such as low and high angle grain boundaries. It is also difficult to maintain the single crystal structure in regions where there are large geometric changes in the casting, for example in the fillet region where the airfoil transitions to the platform. Casting defects such as ‘freckle chains’ are often observed. Material requirements of the blade and platform are different. The blade must tolerate high temperatures and corrosive gas flow. The platform does not reach the highest temperatures of the blade, but needs strength and castability.
Forming the blade and platform as a single piece does not allow material optimization. However, forming them of separate pieces involves fastening, close tolerances, stress concentrations, and vibration issues. U.S. Pat. No. 7,080,971 shows a platform attached to a blade by a pin inserted through a hole passing completely through the platform and shank. This causes stress concentrations.